Boiling water reactors (BWR) have measures to reduce reactor power by scramming the reactor before fuel integrity is damaged. As one of the measures to do this, an oscillation power range monitor (OPRM) for detecting neutron flux oscillations caused by nuclear thermal hydraulic instabilities is used to evaluate reactor core stability (for example, see Japanese Patent No. 3064084).
For example, an advanced BWR (ABWR) has 208 local power range monitor detectors (LPRM detectors) disposed in the reactor core to detect neutron fluxes. These LPRM detectors are grouped into pairs of 4 LPRM detectors and each pair is housed in an instrument tube arranged in the vertical direction, resulting in a total of 52 instrument tubes being disposed in the reactor core.
The ABWR has four oscillation power range monitors. Each oscillation power range monitor is configured to receive neutron flux signals (LPRM signals) from the 52 LPRM detectors of the total of 208 LPRM detectors.
Each LPRM signal contains oscillations caused by noise components other than oscillations caused by nuclear thermal hydraulic instabilities, and thus undergoes filtering to remove the noise components.
The filtered plurality of LPRM signals (filtered flux values) are allocated to a predetermined cell so as to maintain redundancy, and the average flux values averaged for each cell are calculated. Note that an ABWR has 44 cells.
The time averaged flux value is derived by executing time averaging process using a filter with a relatively long time constant on the average flux value. Further, the average flux value is divided by the time averaged flux value to calculate a normalized value with only the oscillation components of the LPRM signal extracted.
Then, the oscillation power range monitor derives the amplitude and the cycle of an oscillating waveform by detecting crest and trough peaks of an oscillating waveform of the normalized value and monitors power oscillations caused by nuclear thermal hydraulic instabilities based on various algorithms. Thus, when a determination is made that fuel integrity may be damaged, a scram signal is generated.
In addition, the OPRM receives an average value (APRM value) of the LPRM signals of the 52 LPRM detectors and a reactor core flow value (FLOW), and determines whether or not the reactor is in an operation area having a possibility that the reactor may generate power oscillations. When a determination is made that the reactor is in the operation area having no possibility that the reactor may generate power oscillations, the OPRM stops the power oscillation monitoring algorithm in order to prevent the scram signal from being generated due to an incorrect determination.
The aforementioned normalized value calculation in the OPRM includes a calculation of the normalized value from the remaining LPRM signals allocated to each cell except the LPRM signals corresponding to the following exceptional conditions (1) to (3).
(1) The generating LPRM detector is in a failure state. (2) An error occurs in the transmission path of the LPRM signal in a preceding stage of the OPRM. (3) The LPRM signal value is smaller (for example, less than 5%) than a preset value.
Note that there may be a case in which during the normalized value calculation, any one of the LPRM signal changed to correspond to exceptional conditions or any one of the LPRM signal changed to not correspond to exceptional conditions. If that happens, the LPRM signals subjected to the normalized value calculation may be removed from or returned to the calculation during the calculation. Therefore, the average flux values change in a discontinuous manner and the accordingly calculated normalized values change in an abnormal manner (see FIG. 4A). Thus, unfortunately, incorrect crest and trough peaks may be detected from the oscillating waveform of the obtained normalized value, leading to an incorrect determination in the power oscillation monitoring.
In addition, when the oscillation power range monitor returns to normal operating state from a state different from the normal operating state such as a bypass, a failure state and a test mode, the same case as the aforementioned case may be assumed to occur. For example, in a test mode, the normalized value calculated at the time when the calculation target is changed from a simulation signal generated outside or inside to an LPRM signal may be discontinuous, leading to an incorrect determination in the power oscillation monitoring.
In view of the above circumstances, the present invention has been made, and an object of the embodiments described herein are to provide a highly reliable reactor power oscillation monitoring technique for suppressing a discontinuous change in extracted oscillation components when an averaging process is performed on a plurality of LPRM signals the number of which to be calculated may be changed.